Virtually all automotive vehicles have an engine (or electric motor) that powers two wheels on a common axle via a transmission and differential (for front wheel drive vehicles the differential is often referred to as a transaxle). When a vehicle has a driven wheel stuck in snow, ice or other low friction surface, the wheel can start to spin. Wheel spin causes the power from the engine to be delivered to the spinning wheel, while the wheel positioned on a superior traction surface is not powered due to the inherent action of the differential. To inhibit the above noted phenomena vehicles have been equipped with traction control systems. Traction control typically works by having a controller signal a solenoid valve to automatically selectively connect a vehicle source of pressurized brake fluid with a wheel cylinder to lock the spinning wheel, thereby causing power to be delivered to the wheel with the superior tractive surface underneath. Additionally traction control also may throttle output of the vehicle's engine. If the vehicle is proceeding on an iced surface underneath both driven wheels, the traction control system selectively powers both wheels to a point wherein the wheel does not reach a wheel slip condition, and then alternately locks the wheel, and then again power the wheel, allowing the vehicle to inch forward.
To provide superior traction than that provided by an automotive vehicle with traction control, many vehicles have four wheel drive. Most four wheel drive vehicles have a primary axle that always powers vehicle, and a secondary axle that can be manually engaged to be powered, or can be powered automatically upon sensing of a wheel slip condition on the primary axle. Four-wheel-drive vehicles require two drive shafts, two differentials and a transfer case. The addition of an extra drive shaft, extra differential and transfer case on the vehicle provides a weight penalty (affecting vehicle fuel efficiency) and a cost penalty for added components. The addition of a transfer case often requires the vehicle frame to be raised upward for road clearance purposes. Accordingly, many four wheel drive vehicles are light trucks or sport-utility type vehicles. It is desirable to provide a traction control system for automotive vehicle that additionally provides some of the benefits of a four wheel drive system without the weight penalty of an additional drive shaft, differential and transfer case. It is also desirable to provide the above noted traction control system while adding minimal costs to the vehicle, allowing the traction control system to be utilized on smaller front or rear wheel drive vehicles in a lower cost range. It is also desirable provide the above noted traction control system in vehicles having a relatively low road clearance.